1. Field of the Invention
The present invention relates to a device for improving amplitude imbalance occurring between two output terminals of an on-chip transformer balun, and more particularly, to an on-chip transformer balun device improving the amplitude imbalance by controlling an inter-winding capacitance value by designing an asymmetrical structure in which one of the primary winding and secondary winding is formed of a plurality of metal layers in which a spiral trace portion, excluding an underpass, is disposed on mutually different layers.
2. Description of the Related Art
A balun is a transformer for matching, used for coupling a circuit that is balanced with the ground with an amplification circuit whose end is grounded, for preventing ground balance of a balanced circuit from being broken, and for connecting a circuit balanced with the ground to an unbalanced circuit, such as a coaxial cable, in a transmission circuit in a microwave band, and functions as a device mutually transforming balanced/unbalanced signals.
A Marchand balun is generally used in a form of an off-chip device in microwave applications, designed to have a length of ¼ of an electric wave of a corresponding frequency, thereby increasing an area occupied on a wafer and increasing manufacturing cost when applied as an on-chip device associated with application in a band less than several GHz.
FIG. 1A is a diagram illustrating structures of a conventional Marchand balun 110 and FIG. 1B is diagram illustrating a conventional overlay transformer balun 120. Generally, in an on-chip device, a balun applying a transformer is used and may achieve similar performance with an area smaller than the Marachand balun 110.
As structures of the transformer balun, there are a planar type and an overlay type. Since most complementary metal oxide semiconductor (CMOS) foundries provide a primary winding 121 and a secondary winding 122, which have multi-metal layers as a form of the overlay transformer balun 120, these are most economical.
However, deterioration of performance due to parasitic capacitance occurs in the overlay transformer balun 120.
FIG. 2A is a diagram illustrating a structure of a conventional on-chip transformer balun 210, and deterioration of performance, caused by the structure. As shown in FIG. 2, the conventional on-chip transformer balun 210 including the overlay transformer balun 120 includes a primary winding 211 and a secondary winding 212.
In this case, the primary winding 211 is connected to a first port 213 and a ground 214, and the secondary winding 212 is connected to a second port 216 and a third port 217, based on a center tap 215.
In this case, in a 1:n transformer compact model, although an input signal inputted via the first port 213 is transformed and transmitted to output terminals of the second port 216 and the third port 217 by magnetized coupling between the primary winding 211 and the secondary winding 212, actually, performance is deteriorated by unexpected coupling in addition to the magnetizing coupling, occurring due to parasitic capacitance as frequency becomes high.
The on-chip transformer balun 210 forms inverting and non-inverting connections due to a property of signal transmission between input and output devices. According to the inverting and non-inverting connections between input and output devices, an effect of the coupling between the primary winding 211 and the secondary winding 212, occurring due to the parasitic capacitance, shows asymmetry with low-pass filter and band-pass filter effects, respectively, thereby increasing amplitude imbalance between two output terminals as a frequency becomes higher. A graph 220 illustrated in FIG. 2B shows an effect of the parasitic capacitance 221 via a relation between transmission coefficient magnitude displayed in dB, and a frequency. As shown in the effect of the parasitic capacitance, as the frequency becomes higher, a difference between the transmission coefficient magnitudes becomes greater.
As described above, in the conventional on-chip transformer balun 210, since the amplitude imbalance between the two output terminals rapidly becomes greater as the frequency becomes higher, a usable frequency bandwidth is limited to a low frequency band.